1. Field of the Invention
The present invention relates to an organic electroluminescent device (hereinafter, referred also to as an xe2x80x9corganic EL devicexe2x80x9d), and a group of organic EL devices.
2. Description of the Related Art
Recently, attention is been focused on organic electroluminescent devices having a light-emitting or luminescent layer formed from a specific organic compound due such organic electroluminescent devices being able to achieve a large area display device operable at a low driving voltage. To produce an EL device having high efficiency, Tang et al., as is reported in Appl. Phys. Lett., 51, 913 (1987), have succeeded in providing an EL device having a structure in which organic compound layers having different carrier transporting properties are laminated to thereby introduce holes and electrons with a good balance from an anode and a cathode, respectively. In addition, since the thickness of the organic compound layers is less than or equal to 2,000 xc3x85, the EL device can exhibit a high luminance and efficiency sufficient for practical use, i.e., a luminance of 1,000 cd/m2 and an external quantum efficiency of 1% at an applied voltage of not more than 10 volts.
In the above-described high efficiency EL device, Tang et al. used magnesium (Mg) having a low work function in combination with the organic compound which is considered to be fundamentally an electrically insulating material, in order to reduce an energy barrier which may cause a problem during injection of electrons from a metal-made electrode. However, since the magnesium is liable to be oxidized and is unstable, and also exhibits only a poor adhesion to a surface of the organic layers, magnesium was used upon alloying, i.e., by the co-deposition of the magnesium with silver (Ag) which is relatively stable and exhibits good adhesion to a surface of the organic layers.
On the other hand, researchers of Toppan Printing Co. (cf, 51st periodical meeting, Society of Applied Physics, Preprint 28a-PB-4, p.1040) and those of Pioneer Co. (cf, 54th periodical meeting, Society of Applied Physics, Preprint 29p-ZC-15, p.1127) have had developments in the usage of lithium (Li), which has an even lower work function than that of Mg, and alloying lithium (Li) with an aluminum (Al) to obtain a stabilized cathode, thereby embodying a lower driving voltage and a higher emitting luminance in comparison with EL devices using magnesium alloys.
In addition, as is reported in IEEE Trans. Electron Devices, 40, 1342 (1993), the inventors of the present application have found that a two-layered cathode produced by depositing lithium (Li) alone with a very small thickness of about 10 xc3x85 on an organic compound layer, followed by laminating silver (Ag) to the thus deposited Li layer is effective to attain a low driving voltage in EL devices.
Using the above EL devices, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No.63-264692, if a thickness of the organic compound layer is controlled to not more than 1 xcexcm (substantially 0.2 xcexcm or less), it becomes possible to operate the devices at a low voltage which is acceptable for practical use, even if an organic compound which is basically an electrically insulating material is used in the formation of the organic compound layer.
Further, the applicant of this application, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No.10-270171, has discovered that if a metal showing a low work function such as an alkali metal, an alkali earth metal and transition metals including a rare earth metal, and an organic electron-accepting compound are mixed in the predetermined ratio through co-deposition to form an electron injection layer, the resulting EL device can be operated at a low driving voltage regardless of the work function of the cathode. In this EL device, a donor (electron-donating) dopant substance, i.e., metal, capable of acting as a reducing agent for the organic compound is previously doped into a layer of the organic compound to be contacted with the cathode, and thus the organic compound is retained as a molecule in the reduced form; namely, the molecule of the organic compound has electrons accepted or injected therein. As a result, an energy barrier in the electron injection from a cathode to an organic compound layer is reduced, thereby ensuring a low-voltage driving of the EL devices in comparison to the prior art EL devices. Moreover, in the formation of the cathode, it is possible to use any stable metals which are conventionally used as a wiring material such as aluminum (Al). Accordingly, if a suitable combination of the organic compound and the metal is applied to the metal doping layer, an increase of the driving voltage can be prevented in contrast to the prior art layer constituted from only an organic compound, and such effects can be obtained even if a layer thickness of the metal doping layer is increased to a level in the orders of micrometers. Namely, in this EL device, a dependency of the driving voltage upon layer thickness of the metal doping layer can be removed.
Referring again to the above-described EL device developed by Tang et al., an indium-tin-oxide (ITO) is coated as an anode electrode over the glass substrate. However, the use of the ITO anode electrode in the device taught by Tang et al. to obtain a good contact near to ohmic contact is considered to be made due to unexpected luck, because, in the hole injection to the organic compound, the ITO electrode has been frequently used as a transparent anode electrode made of metal oxide to satisfy the requirement for the emission of light in the planar area, and the ITO electrode can exhibit a relatively large work function of not more than 5.0 eV.
Further, in the EL device taught by Tang et al., a layer of copper phthalocyanine (hereinafter, CuPc) having a thickness of not more than 200 xc3x85 is inserted between the anode and the hole-transporting organic compound layer to further improve the contact efficiency of the anode interface region, thereby enabling the operation of the device at a low voltage.
Furthermore, the researchers of Pioneer Co., Ltd., have obtained similar effects by using a starburst type arylamine compounds, proposed by Shirota et al., of Osaka University.
CuPc compounds and starburst type arylamine compounds have characteristics that show a work function smaller than that of ITO and a relatively high mobility of the hole charge; and thus they can improve the stability of the EL devices during continuous drive, as a function of improved interfacial contact, in addition to low-voltage driving.
On the other hand, the applicant of this application and others, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No.10-49771, have discovered that, if a Lewis acid compound and an organic hole-transporting compound are mixed in a predetermined ratio using a co-deposition method to form a hole injection layer, the resulting EL device can be operated at a low driving voltage regardless of the work function of the anode. In this EL device, a Lewis acid compound capable of acting as an oxidation agent for the organic compound is previously doped into a layer of the organic compound to be contacted with the anode, and thus the organic compound is retained as a molecule in an oxidized form. As a result, an energy barrier in the hole injection from an anode to an organic compound layer is reduced, thereby ensuring a low-voltage driving of the EL devices in comparison to the prior art EL devices. Accordingly, if a suitable combination of the organic compound and the Lewis acid compound is applied to the hole injection layer, an increase of the driving voltage can be avoided in contrast to the prior art layer constituted from only an organic compound, and such effects can be obtained even if a layer thickness of the hole injection layer is increased to a level in the order of micrometers. Namely, in this EL device, a dependency of the driving voltage upon the layer thickness of the hole injection layer can be removed. Details of this EL device should be referred to a preprint of 47th periodical meeting of Japanese Society of Polymer, Vol.47, No.9, p.1940 (1998).
In addition, there have been made other approaches to improve the organic EL devices, because an emission spectrum of the EL devices relies upon the fluorescence generated by the organic dyes, and thus a half-width thereof is generally large. The large half-width of the emission spectrum is insufficient to satisfy the requirements for tone control of the devices.
As is disclosed in Japanese Unexamined Patent Publication (Kokai) No.8-213174, Nakayama et al., of Hitachi Ltd., have succeeded in giving an optical resonator function to the EL device, thereby improving the purity of color of light emitted from the device. The invention taught provides a translucent reflective layer between a glass substrate and a transparent ITO electrode to control an optical distance (length of optical path) between a light emission layer and a back electrode (anode).
The layer structure similar to that of Nakayama et al., was also adopted by Tokitoh et al., of Kabushikikaisha Toyota Chuo Kenkyusho. Namely, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No.9-180883, Tokitoh et al. have determined a length of the optical path using a similar layer structure to obtain a single emission mode, thereby ensuring a monochromaticity and a strong directivity in a front direction.
As will be appreciated, both of the above EL devices have a layer structure in which a translucent reflective layer is sandwiched between a transparent electrically conducting layer as an anode and a glass substrate, the translucent reflective layer being formed by alternately depositing thin layers having different indexes of refraction such as TiO2 and SiO2 with sputtering or the like, and an optical resonator structure is formed between the reflective layer and the anode as a reflecting mirror. However, when it is intended to form a charge injection layer in these EL devices by using only an organic compound as in the prior art organic EL devices, to obtain a effective length of optical path sufficient to utilize an interference effect of light, it is necessary to provide a translucent reflective layer in accordance with the above-mentioned manner, in addition to formation of the organic layer.
The present invention is designed to solve the above-mentioned problems of the EL devices of the prior art, wherein an object of the present invention is to provide an EL device wherein the driving voltage can be reduced by forming an electron injection layer to be contacted with a cathode as a metal doping layer, or by forming a hole injection layer to be contacted with an anode as a chemical doping layer; and, at the same time, to utilize the lack of dependency of the driving voltage upon the layer thickness of the metal doping layer or chemical doping layer, to thereby function as an emission spectrum controlling layer to the electron injection layer or hole injection layer.
In order to achieve the above-mentioned object, an organic electroluminescent device is provided, including at least one luminescent layer, constituted from an organic compound, provided between a cathode electrode and an anode electrode opposed to the cathode electrode; and an organic compound layer doped with a metal capable of acting as an electron-donating dopant, the organic compound layer being disposed as a metal doping layer in an interfacial surface with the cathode electrode. An emission spectrum of light emitted from the organic electroluminescent device is controlled by varying a layer thickness of the metal doping layer.
Preferably, the metal doping layer includes at least one metal selected from an alkali metal, an alkali earth metal and transition metals including a rare earth metal, the metal having a work function of not more than 4.2 eV.
Preferably, the metal is included in the metal doping layer by a molar ratio of 0.1 to 10 with respect to the organic compound.
Preferably, the metal doping layer has a layer thickness of not less than 500 angstroms.
In an embodiment, the organic compound in the metal doping layer can act as a ligand to an ion of the metal in the metal doping layer.
In an embodiment, the metal doping layer includes divided areas having different layer thicknesses.
Preferably, the divided areas each includes a group of picture elements arranged in a matrix form.
Preferably, each of the divided areas has a controlled layer thickness to obtain a specific emission spectrum in each divided area.
According to another aspect of the present invention, a group of organic electroluminescent devices are provided, each organic electroluminescent device including at least one luminescent layer, constituted from an organic compound, provided between a cathode electrode and an anode electrode opposed to the cathode electrode; and an organic compound layer doped with a metal capable of acting as an electron-donating dopant, the organic compound layer being disposed as a metal doping layer in an interfacial surface with the cathode electrode. A layer thickness of the metal doping layer is controlled in each organic electroluminescent device so that light emitted from the each organic electroluminescent device has different emission spectrums.
Preferably, the metal doping layer includes at least one metal selected from an alkali metal, an alkali earth metal and transition metals including a rare earth metal, the metal having a work function of not more than 4.2 eV.
Preferably, the metal is included in the metal doping layer by a molar ratio of 0.1 to 10 with respect to the organic compound.
Preferably, the metal doping layer has a layer thickness of not less than 500 angstroms.
Preferably, the organic compound in the metal doping layer can act as a ligand to an ion of the metal in the metal doping layer.
According to another aspect of the present invention, an organic electroluminescent device is provided, including at least one luminescent layer, constituted from an organic compound, provided between a cathode electrode and an anode electrode opposed to the cathode electrode; and an organic compound layer doped with an electron-accepting compound, having properties of a Lewis acid, disposed as a chemical doping layer in an interfacial surface with the anode electrode on the luminescent layer side. An emission spectrum of light emitted from the organic electroluminescent device is controlled by varying a layer thickness of the chemical doping layer.
In an embodiment, the chemical doping layer is a layer of organic compound formed upon doping, through co-deposition in a vacuum, of the electron-accepting compound.
In another embodiment, the chemical doping layer is prepared by reacting an organic compound and the electron-accepting compound in a solution.
Preferably, the organic compound constituting the organic compound layer is a polymer.
In an embodiment, the organic compound layer includes the electron-accepting compound by a molar ratio of 0.1 to 10 with respect to the organic compound constituting the organic compound layer.
In an embodiment, the organic compound layer includes the electron-accepting compound by a molar ratio of 0.1 to 10 with respect to an active unit of the polymer constituting the organic compound layer.
Preferably, the chemical doping layer has a layer thickness of not less than 50 angstroms.
In an embodiment, the electron-accepting compound includes an inorganic compound.
Preferably, the inorganic compound includes at least one member selected from a group consisting of ferric chloride, aluminum chloride, gallium chloride, indium chloride and antimony pentachloride.
In another embodiment, the electron-accepting compound includes an organic compound.
In an embodiment, the organic compound includes trinitrofluorenone.
Preferably, the chemical doping layer includes divided areas having different layer thicknesses.
Preferably, each of the divided areas includes a group of picture elements arranged in a matrix form.
Preferably, wherein each of the divided areas has a controlled layer thickness to obtain a specific emission spectrum in each divided area.
According to another aspect of the present invention, a group of organic electroluminescent devices is provided, each organic electroluminescent device including at least one luminescent layer, constituted from an organic compound, provided between a cathode electrode and an anode electrode opposed to the cathode electrode. The organic electroluminescent devices each includes an organic compound layer, as a chemical doping layer, doped with an electron-accepting compound having properties of a Lewis acid, the organic compound layer being disposed in an interfacial surface with the anode electrode on the luminescent layer side of the organic electroluminescent device. A layer thickness of the chemical doping layer is varied in the each organic electroluminescent device so that light emitted from the organic electroluminescent device has different emission spectrums.
In an embodiment, the chemical doping layer is a layer of organic compound formed upon doping, through co-deposition in a vacuum, of the electron-accepting compound.
In another embodiment, the chemical doping layer is a layer of organic compound formed upon coating a coating solution which is prepared by reacting an organic compound constituting the organic compound layer and the electron-accepting compound in a solution.
Preferably, the organic compound constituting the organic compound layer is a polymer.
In an embodiment, the organic compound layer includes the electron-accepting compound by a molar ratio of 0.1 to 10 with respect to the organic compound constituting the organic compound layer.
In an embodiment, the organic compound layer includes the electron-accepting compound by a molar ratio of 0.1 to 10 with respect to an active unit of the polymer constituting the organic compound layer.
Preferably, the chemical doping layer has a layer thickness of not less than 50 angstroms.
In an embodiment, the electron-accepting compound includes an inorganic compound.
In another embodiment, the electron-accepting compound includes an organic compound.
The present disclosure relates to subject matter contained in Japanese Patent Applications No.11-276933 (filed on Sep. 29, 1999) and No.2000-54176 (filed on Feb. 29, 2000) which are expressly incorporated herein by reference in their entireties.